Method of stripping enriched absorbent medium



METHOD OF STRIPPING ENRICHED ABSORBENT MEDIUM Filed Aug. 5l

GLE/V'S. HOUGHLHND.

S rw Sk INVENTOR ATTORNEY Patented Aug. 24, 1943' Msrnon or sTRrPPINd insistenti)y j ABSQRBENT MEmUM: Y cieii'- s. nughiana, New Yoran. Y., as-signor to The-MiW. KlloggComp/amm Jerseyy City, N. kJ.,

2 Claims:

This invention relates to k `the recovery ofv pro-pane and similar lightvhydrocarbons 'from gases by absorption, and is especially concerned Withfa process for stripping recovered hydrocarbons from an oil which haspbeen enriched ther'ef with in anabsorber. 4. p iy The stripping operation lwhich,1n-ust,always complement an absorption step .in a hydrocarf bon recovery process should accomplish thecorn-v plete isolation Vof the yabsorbed'V hydrocarbons from the absorption medium.Y I'the absorption mediumis not thoroughly denudedof recovered hydrocarbons, its absorbing capacity Will be impaired, While the presence of absorption medium in the finally recovered hydrocarbons `is undesirable if only for the reason that it constitutes a lossof medi-umfromwthe process.;`

Heretofore when `propane has been` recovered afeorpor'ationo Delaware v c i Applieautnnugsusrs'i, icitgjseriaino; 3543er .l

by absorption it has been customary tofeondut- Stripping Operations :under pressures. of 20010 Ll0()v pounds per square inch;` which necessitates heating the enriched 4oil to a temperature of 50Q F. or higher if complete removalof light hydrocarbons is to be'accomplished/efficiently- The .25 ysolelyfrn recovered hydrocarbonscompletely free latter temperature level can seldornr be attained except by means of a furnace orjtubefstilLfwhich contributesgreatly` to4 the cost or a stripping ul'lll` A Vj 1 Themain object of my invention is toprovide a method `of stripping enriched: or flat absolrp tion oilwherein the temperatures employedare attainable tv heat @Xchangel with .Steam- .et the commonly v`vavailable pressure ofgl'fwtof- A175 pounds per square inch. ASuch stearnfishusually used for other purpcsesl in the naturalgasoline. plantsand refineries where y hydrocarbonlrecoveryr is practiced, so thatby vmethod-the construction of a furnace as part of theustripping equipment can frequently be avoided. The' relatively low temperatures necessary Witlimygrneth-l od also eliminatel .the need fcorexpensivelhot; oil-circulating pumps and'y numerous heaty ex.- changers,v and retard the deterioration i? the absorption oil due to gum formation.

In general terms, the method ofmy invention comprises subjecting an enriched labsorption."oil

to flash` distillationf to separate. light hydrocarbons from absorption oil roughlyV and" Without intending to obtain either entirely free froI-nthe other. This initial flash may .be e'ected at `any desired-1. pressure] consistent with the require'-v ment that the temperature ofthe oil'be held below about 400 El and-that the bulk the light hydrocarbons b'eftaken overhead.

The liquid rem'aiiing unvaporized'in tl'ie fore` liminaryseparatiom consisting of thefbulk of the absorptionpui man' inc'oihilfeteiy strip d condition, istransferred to an atmospherc or low-pressure fractionating zone where complete Volatiliration of recovered hydrocarbons is efsible to strip the absorption oilcompletely at a relatively low temperature-41101:, however,.-with out'lcarrying light ends of the oilv overhead with the stripped vapors. Y Asl will be'un'd'erstood from theV hereinafter-given. description. of subsequent steps, the quantity Vof lighty ends'thus vaporized is constant i and is continuously returned, so that in actuality the stripped liquid includes the vfhole of the absorption oil charged. This liquid, after cooling, isready to be used-for the absorption. of `:additional quantities of hydrocarbons.` f

Hydrocarbons and light ends stripped out in the preceding lstep are introduced into a high pressurefractionating Zone as-mare the vapors vjfrom. the preliminary iiashseparation. The materialsfenteringthe high pressure ffractionating zone thus include all the recovered hydrocarbons .plus light endsroffthe absorber Oil. At the high 'pressure thel.v1atter materials rmay be excluded frommthefoverhead distillate and are collectedin the bottom product, and the overhead distillate isfreadily .condensed at normal" cooling- Water temperatures. The overhead distillate consists o'faabsorp'tion oily fractions, but thisres'ult is not attained withoutleaving some light hydrocarbons..injtl'ie.bottoms',` Aswill be understoodfrom the further. description which follows, the quanftity of recoveredV hydrocarbons Withdrawn in the bottoms from the high pressure fractionating Yzone isgconvstant and is continuously returned thereto :so that in actuality thefoverhead vapors includegall-gthe recovered hydrocarbons. The said vapors"- are Withdrawn as the desired productY The bottomsy liquid collected inthe high ,pressurel fractionation zone, consisting of lightv fractionsfo absorber o il and a small amount of recovered hydrocarbone,` is then passed tothe low pressure fractionating Zone.` Since therVV liquid Withdrawnirom .thez latter zone is thefstripped abscrbeig oi-l-,gand the vapors therefrom are passed to the highs pressure zone from- Which recovered hydrocarbons are taken, it Willbe seen that,r as

Ypreviously stated, the recycling "of constant Yquantities.n Qfvaporized light ends and condensed light nydrocarbonsyis achieved. `The temperatuiegmaintafined in the `high pressure fraction'- .atingzoneare ofthe'same magnitude as those kin the low-pressureiractionating zone.

The" annexed drawing to which reference `Will new bemade is a diagrammatic flow sheet'of one embodiment of my invention. j

In the drawing, a stream-of absorption' oil enrichedljwith light hydrocarbons fromy an ab- .sorberis introduced into-line. I and passed there- .through to. af flash. or preliminary separator i." The stream. traversing. line .I brought by means of heat exchanger 3V to a temperature not ment takes place.

line i to the top of fractionator I2 as reflux higher than about 400 F., and the pressure in ash drum 4 is made low enough to insure vaporization of the major portion of the light hydrocarbons in the entering oil stream at the chosen temperature. It will be understood that the number of combinations of temperature and pressure capable of satisfying the latter requirement is infinita'and that the higher the temperature-selected, the higher may be the pressure. A temperature between about 150 F. and that of the hottest heating medium which may happen to be available is preferable, but as ,previously mentioned, an upper limit of about 400 F. is adhered to in any case.

Liquid remaining unvaporized in the iiash drum comprising the bulk of the absorption oil in an incompletely stripped condition is collected and withdrawn through line I0 and passed to a fractionatingrcolumn I2 equipped with a reboiler I3,

the latter suitablyr taking ther form of a steamd heated coil in the base of the column. The co1- umn is maintained under a pressure not higher than about V50 pounds per square inch, which is low enough to permit stripping the absorption oil vcompletelyl without employing temperatures above about 400 F. Stripped absorption oil is withdrawn from the base of fractionator I2 through line I4 and when cooled is ready for reuse in an absorber. 1

Overhead vapors from fractionator I2 go Via line I5 to a partial condenser I6 and the therein produced vapor-condensate mixture continues to a reflux accumulator I1 wherein vapor disengage- Condensate is returned via while vapor is withdrawn through line I8 and passed via line 1 to a high-pressure fractionating column 8. Vapors from the flash drum 4 also travel Via line 5 tothe high-pressure column 8. Fractionator 8 will in all cases be at a higher pressure than polumn I2, so that a compressor 6 is necessary between lines I3 and 1 to raise the'pressure of the vapor passing therethrough. lin the event that flash drum 4 is operated at a lower pressure than column 8, a compressor will be necessary to pressure the vapors traversing line 5 also.

Column 8 is provided with a suitable reboiler coil whereby its. bottom temperature is maintained as high as is consistent with the'temperature of the available heating medium but not above about 400 F. This column is maintained under sufficient pressure to enable the desired recovered hydrocarbons to be condensed at cooling water temperature, normally between 200 and 350 pounds per square inch. A

Because of the relatively high pressure in fractionator 8, it is possible to drive the light absorption oil fractions, into the bottoms by means of reflux and to `withdraw an overhead product:

which consists solely of light hydrocarbons. These pass off through line 24 to condenser 25, the condensate being collected in accumulator 21. A portion for reflux purposes is returned through line 29 to the top of fractionator 8 and theremainder withdrawn through line 28 for use as thedesired product. If anyfexcessively light hydrocarbons such as methane and ethane were picked up by the absorption oil along with those desired 'to be recovered they will remain as a vapor above the liquid in the accumulator 21 and as such a vapor would contain small amounts of heavier hydrocarbons it could advantageousl be returned to an absorber via line 30.

The bottoms product from fractionator 8, consisting of light fractions` derived from the absorption oil and heavier recovered hydrocarbons, is

withdrawn through line 2l and after expansion through valve 22 is carried by line 23 back to the low pressure fractionator I2.

Itfwill be seen from the foregoing exemplary embodiment and general description that absorber oil may be stripped by my method in a relatively simple, easily controllable manner which is adaptable to a wide range of hydrocarbon recovery conditions and which employs temperatures attainable by heat transfer from commonly available heating media.

By my method the iinal isolation of light hydrocarbons free from absorption oil and of absorption oil free from light hydrocarbons are carried out in separate steps, each under a pressure especially suited thereto. lIt will be understood that the now sheet showing one embodiment Hof my invention is purely diagrammatic and that thel invention is capable of many refinements which will readily occur to those skilled inthe art; -for example, economy of heat maybe enhanced by exchanging heat between streams of materials being processed. Also, stripping-may be'accomplished more effectively in both the low pressure zones by injecting superheated steam directly into the bottoms products collected therein.

I claim: V

1. A method of separating light hydrocarbons from an absorption oil which has been enriched therewith by contact with a hydrocarbon-containing gas including the steps of ilashingenriched absorption oil to produce a vapor containing most of the light hydrocarbons originally present in the enriched oil but not entirely free from light 'fractions of the oil itself and a liquid containing most of the absorptionl oil but not entirely'free oflight hydrocarbons, passing said liquid to a first fractionating zone and therein stripping substantially all the remaining light hydrocarbons out of said liquid at a temperature' not higher than about 400 F. and under a pressure low enough topermit said stripping at the prevailing temperature, withdrawing stripped absorption oil as a bottoms product from said first zone, withdrawing an overhead vaporous product from said first zone consisting of light hydrocarbons and light fractions of absorption oil, introducing at least a part of said overhead product in vaporous condition and iiash-vaporized material from said ashing step into a second fractionating zone, maintaining'in said second fractionating Zone a pressure higher than that existing in said rst zone, fractionatingsaid` overhead product and ash-vaporized material in said second fractionating zone to obtain a bottoms liquid, including light hydrocarbons and light abi than about 50 pounds per square inchk and theA pressure in said second fractionating zone is between about 200'andabout 350 pounds per square inch.

' GLENS. HOUGHLAND. f 

